As described in Kokai Patent Application No. 2004-56978 and Kokai Patent Application No. 2004-325263, a voltage sensor (or a voltage detector) used to detect the voltage of an assembled battery in which multiple electric cells are electrically connected in series is known. This kind of voltage sensor must be able to detect the total voltage, which is a higher voltage generated by connecting multiple cells in series. This voltage sensor is thus used to detect a wide range of voltage values, and error becomes more significant as a result. Thus, an error correction operation is usually needed in which the total voltage is detected using a high-accuracy tester at the time of shipment, a detection error of the voltage sensor was measured based on this highly-accurate, detected total voltage, and the error for each unit is corrected using a variable resistor. This error correction operation is time-consuming.